Field of the Invention
The present invention relates to dielectric filters.
Description of the Background Art
As a dielectric filter employing a dielectric coaxial resonator, the one disclosed in, for example, Japanese Utility Model Publication No. 62-44566 (H01P1/205) has been conventionally known.
The dielectric filter of this type is structured as follows: one-end face short-circuited type coaxial resonators are disposed on a dielectric substrate. The coaxial resonators each comprise a dielectric member having a through hole formed therein. Each of the coaxial resonators is formed by coating a conductive member, e.g., silver (Ag) over an outer peripheral surface of the dielectric member and over an inner peripheral surface of the through hole. Electrodes formed on the dielectric substrate are capacitance-coupled to the respective coaxial resonators.
In recent years, communication apparatus have been reduced in scale and weight in the field of mobile communication. With the scale and the weight of the communication apparatus decreasing, smaller-scale dielectric filters have been required.
In the manufacture of a smaller-scale dielectric filter, a ratio of an inner coaxial diameter to an outer coaxial diameter must be 3.6 in order to obtain a high Qu value (unloaded Q value). If the outer coaxial diameter is less than or equal to 4 mm, however, the inner coaxial diameter is less than or equal to 1.2 mm. It is thus difficult to insert a member for external connection into the through holes of the coaxial resonators to connect the same with an external circuit as disclosed in the above-described publication.